Cables, such as power (e.g., electric) cables and optical cables, can often experience mechanical stress and strain when they are elongated or bent under tensile forces and torques during operations. For example, electric cables, for example those for heavy-duty applications and/or for mobile installations, such as mobile harbor cranes, ship-to-shore container cranes, ship unloaders, spreaders, and mining and tunneling equipment, are specifically designed to withstand harsh environment conditions and high mechanical stresses, such as the pulling forces (e.g., tensile loads), bending forces, and torques. The resulting strain may be static or dynamic. The resulting strain may be tensile strain caused by the elongation of the cable and/or bending strain caused by the bending of the cable.
Tensile loads, bending/compressive loads, and twisting in a cable, such as a mobile cable, may result from forced guidance of the cable during the winding and unwinding phases around reels or from collection of the cable within baskets (e.g., for spreader cables). Winding and unwinding phases are typically discontinuous and often abrupt, for example when caused by a horizontal movement of a crane, thereby imposing significant dynamic tensile loads on the cable, and thus on the individual conductors within the electric cable. In addition, other systems for cable movement, such as pulley systems and tender systems, generally involve high tensile loads on the electric cable during operation. Fault or dysfunction of the powered apparatus may lead to the misplacement of the cable, which may, for example, drop from the guiding means thereof or be squeezed by apparatus portions thus causing an undue bending of the cable.
Excessive elongation of the electric or optical cables may cause the tensile loads to be transferred to the electrical or optical conductors, causing damage to the electrical or optical conductors. Excessive and/or prolonged tensile loads may result in a permanent elongation of the cable, which would shorten the life of the cable.
Similarly, excessive bending of the cable may cause the compressive loads to be transferred to the electrical or optical conductors, causing damage to the electrical or optical conductors.
Optical fiber strain sensors (hereinafter also referred to as strain sensors) have been used in power (or electric) and optical cables for monitoring strain (e.g., tensile and/or bending strain) occurring in the cables. The optical fiber included in a strain sensor is surrounded by one or more protective layers that provide mechanical protection and transfer to the optical fiber the strain experienced by the conductors of the cable to be monitored. The strain transfer is attained by suitable layers surrounding the optical fiber in a tight configuration.
Conventional tight-buffered optical fibers are designed for providing data communication, not for measuring strain. In conventional tight-buffered optical fibers, when strain increases, the outer buffer layer of a tight-buffered fiber may slip with respect to the coating system of the inner fiber because it is not bonded with the inner fiber tight enough. Thus, in a conventional tight-buffered configuration strain transferability from the cable core to the buffer layer and the optical fiber is unsuitable for cable strain monitoring.
In an optical-fiber strain sensor, a tighter configuration for the surrounding protective layers is typically used. In such a configuration, as disclosed for example in U.S. Pat. No. 9,032,809, the material of a buffer layer may be selected so as to adhere to the coating system of the optical fiber with essentially no or limited creeping, slipping or debonding. When the strain sensor is stripped to remove the buffer layer and expose the optical fiber, such a tighter configuration would typically require a greater strip force than conventional tight-buffered optical fibers.
PCT International Publication WO 2007/107693 (the '693 publication) discloses a fiber optic cable including a central optical fiber 105 that senses the cable stress, and at least one peripheral optical fiber that experiences only a portion of the cable stress imparted to the central optical fiber 105. With the structure disclosed in the '693 publication, a differential response to strain between a central optical fiber and a peripheral optical fiber can purportedly be obtained.
PCT International Publication WO 2010/136062 discloses a power cable provided with a strain sensor comprising an optical fiber coated by a primary coating, which is surrounded by a secondary coating. The primary coating and the secondary coating form a coating system. The optical fiber can be tight-buffered with a buffer layer surrounding the coating system. An adhesion-promoting layer can be provided between the optical fiber coating system and the tight buffer layer.
One problem encountered when using tightly buffered optical fibers is that of accessibility. It is desirable to be able to remove the protective buffer layer quickly, so that the enclosed optical fiber can be readily accessed, and various solutions have been proposed. For example, U.S. Patent Application Publication US 2011/0026889 (the '889 publication) discloses tight-buffered optical fiber units. The optical fiber unit includes an optical fiber that is surrounded by a polymeric buffering layer to define a fiber-buffer interface. The buffering layer includes an aliphatic amide slip agent (e.g., oleamide and erucamide) in an amount between about 0.01 percent and 0.5 percent sufficient for at least some of the aliphatic amide slip agent to migrate to the buffer-fiber interface to thereby promote stripping of the buffering layer. The buffer tube may be formed predominately of polyolefins, including fluorinated polyolefins. At least about 15 centimeters of the polymeric buffering layer can be removed from the optical fiber in a single operation using a strip force of less than about 10 N.
European Publication EP 0 527 266 (EP '266) discloses a tight buffered optical waveguide fiber having improved strippability and including an interfacial layer in contact with the first protective coating, and a buffer layer in contact with the interfacial layer. The interfacial layer comprises a solid lubricant such as sub-micron particles of polytetrafluoro-ethylene in a film-forming binder. This layer generally has a thickness of between about 4 and about 15 microns, and preferably about 5 microns. The solid lubricant comprises more than about 60% by weight of the layer and most preferably at least about 90%. The composition of the film-forming binder is chosen so that the binder will have an adequate level of adherence to the first protective coating so as to hold the solid lubricant in place on that coating.
EP '266 discloses that the interfacial layer is preferably more adherent to the first protective coating than to the buffer layer. In this way, the buffer layer can be readily stripped from the fiber, leaving behind the interfacial layer and the first protective coating.
The Applicant faced the problem of providing a strain sensor comprising an optical fiber suitable for monitoring the strain (tensile and/or bending strain) experienced by the conductors of an electric or optical cable in which it is located. In particular, Applicant confronted the dilemma of providing a cable strain sensor with a mechanical congruence with the cable conductors sufficient for monitoring cable strain while having characteristics enabling the optical fiber of the sensor to be released from the protecting layers without damaging the fiber and without much difficulty for the operator.
For connecting the optical fiber of the strain sensor to a measurement apparatus, a portion of the optical fiber should be exposed by stripping off the layers surrounding the optical fiber. But due to the high strip force required because of the tight configuration of the layers, the removal of the layers may be difficult and may cause damage to the optical fiber or alter the properties of the optical fiber. For example, the coating system of the optical fiber of the strain sensor may be damaged or the physical properties of which may be altered during the stripping process.
Applicant has therefore tackled the problem of releasing portions of an optical fiber in a mechanically coupled strain sensor during stripping without causing damage to the underlying fiber. To improve the strippability such that when the protective layers are removed no damage or change of properties will occur to the optical fiber included in the strain sensor, one method is to reduce the adhesion between the optical fiber and the protective layers using a slip agent or other lubricant materials. On the other hand, reducing the adhesion too much may lessen the mechanical congruence between the optical fiber (i.e., the strain sensing element) and the cable core (e.g., an electric conductor of an electric cable or an optical fiber of an optical cable) that is being monitored for the strain it experiences. As a result, the strain experienced by the cable core may not be sufficiently transferred to the strain sensor, and thus the strain measured by the strain sensor may not reflect the actual strain experienced by the cable core.